ppmhist: add page (#10950)

* ppmhist: add page

---------

Co-authored-by: K.B.Dharun Krishna <kbdharunkrishna@gmail.com>

pages/common/ppmhist.md

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+# ppmhist
+
+> Print a histogram of the colors present in a PPM image.
+> More information: <https://netpbm.sourceforge.net/doc/ppmhist.html>.
+
+- Generate the histogram for human reading:
+
+`ppmhist -nomap {{path/to/image_file.ppm}}`
+
+- Generate a PPM file of the colormap for the image, with the color histogram as comments:
+
+`ppmhist -map {{path/to/image_file.ppm}}`
+
+- Display version:
+
+`ppmhist -version`